Cytotoxic T lymphocyte responses play a major role in the control of human immunodeficiency virus type 1 (HIV-1) infections; however, most HIV-1-infected patients eventually are unable to efficiently control viral replication and succumb to their disease. The reasons for this eventual failure of virologic control are not well understood. One possibility is that the constant antigen stimulation in the context of a weak CD4-helper response as seen in chronic HIV infection leads to a loss of CD8+ T cell lytic activity by donal deletion or anergy. The first specific aim hypothesizes that within the same individual, distinct populations of HIV-1 specific CD8+ T cells are maintained which vary in their functional characteristics. If our hypothesis were correct, then we would expect to detect virus specific CD8+ T cells that fail to produce interferon-g (IFN-g). To experimentally address this hypothesis, we will comprehensively analyze peripheral blood mononuclear cells (PBMC) from HIV-1 infected subjects using tetramer and intracellular and secretory cytokine staining. The second SA hypothesizes that functional differences seen in HIV-1 specific CD8+ T cells are an important factor in determining disease progression. If our hypothesis were correct, then we would predict that the clearance of HIV-1 infected cells, plasma viral load, and the development of cytotoxic T lymphocyte (CTL) escape mutations correlates with virus specific CD8+ T cell functional heterogeneity. We will test this hypothesis by comparing the functional heterogeneity of virus specific CD8+ I cells with markers of HIV-1 disease progression and epitope specific CTL escape mutations to determine a correlation. Our final SA hypothesizes that HIV-1 specific CD8+ T cells require CD4 help and the level of required help varies depending on the amount of MHC bound peptide. If this hypothesis were correct, then we would predict that low levels of cell surface peptide concentrations correlate with poorly functioning CD8+ T cells specific for that epitope. We Will test this hypothesis by comparing HIV-1 specific MHC peptide concentrations on the surface of CD4+ T cells with the function of HIV-1 specific CD8+ I cells ex vivo to demonstrate that CD8+ T cells activated with relatively low concentrations of antigen require greater CD4 help for optimal function. We will also incubate PBMC or CD8+ I cells with CD4 helper cytokines in order to change the functional phenotype, thereby supporting the notion that CD4 helper responses augment virus specific CD8+ T cells. Findings from this study could lead to better therapeutic options for patients infected with HIV-1 and allow for prolonged discontinuation of antiretroviral treatment.